This invention relates in general to electric motors and in particular to an improved structure for a variable reluctance electric motor.
Electric motors are well known devices which convert electrical energy to rotary mechanical energy. To accomplish this, electric motors establish and control electromagnetic fields so as to cause the desired rotary mechanical motion. There are many different types of electric motors, each utilizing different means for establishing and controlling these electromagnetic fields. Consequently, the operating characteristics of electric motors vary from type to type, and certain types of electric motors are better suited for performing certain tasks than others.
Synchronous motors constitute one principal class of electric motors. The two basic components of a synchronous motor are (1) a stationary member which generates a rotating electromagnetic field, generally referred to as the stator, and (2) a rotatable member driven by the rotating magnetic field, generally referred to as the rotor. Synchronous motors are characterized in that the rotational speed of the rotor is directly related to the frequency of the electrical input signal applied thereto and, therefore, the rotational speed of the electromagnetic field generated thereby. Thus, so long as the frequency of the applied electrical input signal is constant, the rotor will be driven at a constant rotational speed. Within this broad definition, however, the structure and operation of synchronous electric motors vary widely.
One variety of synchronous electric motor is known as a variable reluctance motor. Variable reluctance motors operate on the principle that a magnetic field which is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to become aligned with the magnetic flux (lines of force) generated by the magnetic field. Thus, by using the stator to establish and rotate a magnetic field about a rotor formed from a magnetically permeable material, the rotor can be driven to rotate relative to the stator. The resistance to the passage of this magnetic flux from the stator to the rotor is referred to as reluctance. The magnitude of this reluctance changes with the rotational position of the rotor relative to the stator. Thus, electric motors of this type are commonly referred to as variable reluctance motors.
In a basic variable reluctance motor structure, this operation can be accomplished by providing a generally hollow cylindrical stator having a plurality of radially inwardly extending poles formed thereon. A winding of an electrically conductive wire is provided about each of the stator poles. Concentrically within the stator, a cylindrical rotor is rotatably supported. The rotor is provided with a plurality of radially outwardly extending poles. However, no electrical conductor windings are provided on the rotor poles. By passing pulses of electrical current through each of the stator windings in a sequential manner, the stator poles can be selectively magnetized so as to attract the rotor poles thereto. Consequently, the rotor will rotate relative to the stator.
To optimize the operation of the variable reluctance motor, the magnitude of the electrical current which is sequentially passed through the stator windings is typically varied as a function of the rotational displacement of the rotor, as opposed to simply being supplied in an on-off manner. For example, the magnitude of the electrical current passed through a particular stator winding can initially be large, but decrease as the rotor pole rotates toward it. Consequently, the stator winding is prevented from continuing to attract the rotor pole toward it when the rotor pole has rotated to a position near or adjacent to the stator pole. This facilitates the rotation of the rotor at a more uniform speed.
When selecting any kind of electric motor for use in a particular application, several basic considerations are important. One such basic consideration is the efficiency of the motor, i.e., the ratio of the mechanical output power (torque in rotary electric motors and force in linear electric motors) to the electrical input power. A second consideration is the maximum amount of torque or force which can be generated by the electric motor. A third consideration is the physical size of the electric motor. Obviously, it would be desirable to increase the efficiency and output torque of an electric motor, while reducing (or at least not increasing) the physical size thereof.